Add easy way to specify latest time period only

## Is your feature request related to a problem? Please describe.

A lot of use cases just want us to get the latest time_period of data, to do that currently you'd need to do something like `dplyr::filter(data, period = max(period))`.

## Describe the solution you'd like
Something simple like `time_period = "latest-only"` would work nicely?

## Describe alternatives you've considered
Not thought through other options, might be better ones!

## Additional context
We have a range of time periods, it might be possible just to `max()` it, but also need to test edge cases just in case more fancy / specific logic is needed!

https://api.education.gov.uk/statistics/docs/reference-v1/schemas/TimePeriodCode/#timeperiodcode